Theoretical Study On Solubility Of High CO₂ Content Natural Gas In Formation Water At High Temperature And High Pressure

The reserve of high CO2-content natural gas is very abundant and widely distributed in the global.The interaction of CO2 and water causes extremely complicated phase behavior change of this kind gas reservoir in the process of exploration.On the other hand,the geological storage of CO2 is one of mitigation measures for greenhouse effect.So,studying the solubility of high CO2-content natural gas in formation water has very important significance for the reasonable and efficient development of gas reservoir and for the evelation of dissolving mechanism of CO2 storage.Although the experimental determination of gas solubility in water is accurate,this approach is time-consuming.Thermodynamics models are usually used to predict solubility,which will encounter some challenges because of the physical-chemical effect between CO2 and water.To solve this problem,we adopt theoretical method to study on the solubility model of CH4/CO2 in water,and the main achievements in this paper are described as follows:(1)This paper analyzed the evaluation methods of gas-liquid equilibrium models,chosed the suitable cubic equation of state,mixing rules and activity coefficient models,and derived the fugacity coefficient of PR equation combined with different mixing rules.(2)This paper established a new thermodynamics model(PRSV-MHV1)to predict CO2 solubility in the pure water by combining with PRSV equation of state,MHV1 mixing rule and NRTL activity coefficient model,optimized the interaction parameters,and compared with other thermodynamics models.Under the condition of less binary interaction parameters(?ij,?ji and ?),PRSV-MHV1 model not only has the highest prediction accuracy(AARD=4.02%),which is higher than PR-HV model and PRSV-WS model,but also has a wide range of temperature and pressure(T=273.15?573.15K,P?150MPa).The prediction accuracy of UMR-PRU model is the lowest.(3)Based on particle swarm optimization algorithm,this paper modified the interaction parameters of PR-HV model which was applied to CH4-H2O system,and compared with Duan model and PVTsim software(PR-HV).In the range of T=283.13?523.15K and P<160MPa,the prediction accuracy of modified PR-HV model is slightly higher than that of Duan model and the calculation results of PVTsim(PR-HV)is worst.(4)This paper modified the interaction parameters of PR-HV model and improved the calculation accuracy of the solubility of CO2/CH4 gas in NaCl/CaCl2,solution on the basis of particle swarm optimization algorithm.In comparison with other models,it was found that in the range of T=303-523K,P<150MPa and m=0.173-6mol·kg-1,the prediction accuracy of modified PR-HV model which applied to CO2-H2O-NaCl system,is higher than that of Duan model,and the calculation results of PR-HV model is worst.In the range of T=371.93?513.15K,P?160MPa and m=0.051?5.046mol·kg-1,modified PR-HV model has the highest prediction accuracy for CH4 solubility in NaCl solution,however,Duan model and PVTsim software(PR-HV)only apply to the concentration of solution of 0.051?2.843mol·kg-1 and 1.778-2.843mol·kg-1 respectively.In the end,the average absolute relative error of PVTsim software(PR-HV)for CO2 solubility in CaCl2 solution reach up to 13.56%,which is much higher than that of modified PR-HV model(AARD=3.38%)when the concentration of CaCl2 solution is 1.012,2.28,3.898mol·kg-1.